1. Field of the Invention
The present invention relates to an image formation method for a photothermographic material, in particular, an image formation method for a photothermographic material for scanners or image setters suitable for photomechanical process. More precisely, the present invention relates to an image formation method for a photothermographic material that does not cause physical deformation and uneven density and can provide performances of high contrast and high maximum density.
2. Description of the Related Art
Various image formation methods are known in which image formation is attained by exposing imagewise a photosensitive material having a photosensitive image-forming layer on a support. Among such methods, methods of forming images by subjecting a photosensitive material to heat development are known as methods that contribute to environmental protection and enable simplification of image formation system. In recent years, reduction of amount of waste processing solutions is strongly desired in the field of photomechanical process from the standpoints of environmental protection and space savings. Therefore, techniques relating to photothermographic materials for use in photomechanical process are required to be developed, which enables efficient exposure by a laser scanner or a laser image setter and formation of a clear black image having high resolution and sharpness. Such photothermographic materials can provide users with a simple and non-polluting heat development processing system that eliminates the use of solution-type processing chemicals.
Methods for forming images by heat development are described in, for example, U.S. Pat. Nos. 3,152,904, 3,457,075 and D. Klosterboer, Imaging Processes and Materials, “Thermally Processed Silver Systems A”, Neblette, 8th ed., Chapter 9, page 279, compiled by J. Sturge, V. Walworth and A. Shepp, (1989). Such a photothermographic material contains a reducible non-photosensitive silver source (e.g., silver salt of an organic acid), a photocatalyst (e.g., silver halide) in a catalytically active amount, and a reducing agent for silver, which are usually dispersed in an organic binder matrix. The photosensitive material is stable at an ambient temperature, but when the material is heated at a high temperature (e.g., 80° C. or higher) after light exposure, silver is produced through an oxidation-reduction reaction between the reducible silver source (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image generated upon exposure. The silver produced by the reaction of the reducible silver salt in the exposed region shows black color and this presents a contrast to the non-exposed region to form an image.
Such a photothermographic material is produced by applying coating solutions prepared by dissolving various materials in solvents on a support to form multiple layers including an image-forming layer. As solvents of the coating solutions, organic solvents such as methyl ethyl ketone and acetone may be used. Although a photosensitive material using an organic solvent as a solvent of coating solution may suffer from fluctuation of density due to fluctuation of development temperature or with time, generation of such an event may be suppressed by making the amount of remaining solvent after the coating constant as disclosed in Japanese Patent Laid-open Publication (Kokai, hereinafter referred to as JP-A) No. 6-301140 etc.
By the way, the image formation methods based on heat development comprise a thermal development step in which temperature of photothermographic material exposed imagewise is increased for development. As embodiments of heat development apparatus, there are heat development apparatuses in which a photothermographic material is brought into contact with a heat source such as heat roller or heat drum as disclosed in Japanese Patent Publication (Kokoku, hereinafter referred to as JP-B) No. 5-56499, JP-A-9-292695, JP-A-9-297385 and International Patent Publication WO95/30934, and heat development apparatuses of non-contact type as disclosed in JP-A-7-13294, WO97/28489, WO97/28488 and WO97/28487.
On the other hand, a photosensitive material that provides images of high contrast is required for printing plate making, and uses of hydrazine derivatives are known as techniques of obtaining high contrast as disclosed in U.S. Pat. Nos. 5,545,505 and 5,464,738. However, when these techniques for obtaining high contrast are employed, unevenness of density may be generated by the heat development. Further, since films for printing plate making have a large size, they suffer from a problem of being more likely to cause unevenness of density. Furthermore, they also suffer from a problem that, when uneven external force is applied to them at the time of the heat development, they cause physical deformation and thus unevenness of density is generated. As for the type of heat development apparatus, those of contact type are characterized in that they provide uniform heat development, and those of non-contact type are characterized in that they hardly cause physical deformation since they do not utilize captive force at the time of transportation. However, films for use in printing have a large film size and hence they are more likely to suffer from the problems of unevenness of density and physical deformation. Therefore, there has been desired an image formation method that does not cause physical deformation and unevenness of density and can provide performances of high contrast and high maximum density.